With a conventional turbo-compressor used, for example, in a chemical plant, in many cases, a discharge pressure is set to be almost constant. In such a turbo-compressor, however, the discharge pressure changes if temperature and/or pressure of a suction gas is changed, even when the rotational speed is constant. As a result it may be impossible to reach a predetermined discharge pressure. Then, as is described in Japanese Patent Laying-Open No. Sho 56-121898 (1981), for example, detection is made of the intake temperature and pressure of a working gas, as early as possible, so as to change the rotational speed of a driving machine in response to the intake temperature and pressure detected, thereby controlling the turbo-compressor so that the discharge pressure thereof arrives at the predetermined pressure.
Another example of a conventional turbo-compressor is described in Japanese Patent Laying-Open No. Hei 1-200095 (1989), for example. Within the multi-staged centrifugal compressor described in that publication, the minimum rotation speed of the driving machine is changed or altered in response to changes of temperature of the suction gas, so as to prevent it from an unstable phenomenon, i.e., so-called surging, so that the compressor is operated over a wide range with stability. As is also described, for example, in Japanese Patent Laying-Open No. Hei 10-89287 (1998), the temperature of a working gas sucked into a turbo-compressor is detected so as to change the rotational speed of the compressor in relation to about ⅓ power of a ratio between the detected temperature of the suction gas and a reference temperature thereof, thereby lowering the power of a shaft under a constant gas pressure and keeping the discharge pressure from the turbo-compressor constant.
In contrast to those conventional turbo-compressors, in each of which the constant gas pressure control is obtained by changing the rotational speed of the compressor, in Japanese Patent Laying-Open No. Sho 62-96798 (1987), for example, a change is described as made upon an angle of a vane of an inlet guide vane, which is provided at a suction side of the compressor, corresponding to the temperature of the gas sucked into the turbo-compressor so as to adjust a flow rate with high accuracy.
However, within turbo-compressors such as those which are described in Japanese Patent Laying-Open No. Sho 56-121898 (1981), Japanese Patent Laying-Open No. Hei 1-200095 (1989) and Japanese Patent Laying-Open No. Hei 10-89287 (1998) mentioned above, although the rotational speed of the compressor is controlled depending upon the temperature of the suction gas, in order to change the rotational speed within the turbo-compressor, which is driven by an electric motor, there is a need to provide an electric motor with an inverter drive, resulting in a high price. Also, with a compressor such as that described in Japanese Patent Laying-Open No. Sho 62-96798 (1987), no consideration is paid to the idea of expanding or widening the stable operation region of the compressor when conducting a capacity control thereupon but escaping from surging.
An object of the present invention, accomplished by taking the drawbacks of the conventional technologies mentioned above into consideration, is to provide a turbo-compressor driven by an electric motor, thereby enabling maintenance of a wide operation range or region without generating the surging and while keeping the discharge pressure thereof about constant. Another object, according to the present invention, is to provide a turbo-compressor having a simple structure which is able to maintain a wide operation range or region. According to the present invention, it is also another object to accomplish any one of those objects mentioned above.